1. Field of the Invention
This invention relates generally to an apparatus and method for adjusting the curvature of a mirror in a raster scanning system. More particularly, this invention relates to adjusting the curvature of a mirror to correct for differential bow in a raster scanning system.
2. Description of Related Art
Flying spot scanners (often referred to as raster output scanners or ROSs) conventionally have a reflective multi-faceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a fast scan direction while the recording medium is being advanced in the slow scan direction. The beam scans the recording medium based on a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with the binary sample string. Printers that sweep several beams simultaneously are referred to as multi-beam printers. Both ROS and multi-beam printer techniques are illustrated in U.S. Pat. No. 4,474,422 to Kitamura, the subject matter of which is incorporated herein by reference.
High speed process color or multi-highlight color xerographic image output terminals require multiple independently addressable raster lines to be printed simultaneously at separate exposure stations. This is called multi-station printing. Conventional architectures for multi-station process color printers use a plurality of separate ROSs, usually four independent ROSs, one for each system color as illustrated in U.S. Pat. Nos. 4,847,642 and 4,903,067 to Murayama et al., the disclosures of which are incorporated herein by reference.
The problems with these systems are the high cost of multiple ROSs, the high cost of producing nearly identical multiple ROSs and associated optics, and the difficulty of registering the system colors.
U.S. Pat. No. 5,243,359 to Tibor Fisli, the disclosure of which is incorporated herein by reference, discloses a ROS system suitable for deflecting multiple laser beams in a multi-station printer. FIG. 1 illustrates one embodiment of Fisli's multi-station printer 10. A rotating polygon mirror 12 simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams, having their largest divergent angles parallel to one another. The laser beams are subsequently separated by a plurality of optical filters 16, 18 and 20 and are directed onto respective photoreceptors 24, 26, 28 and 30 using mirrors 21 and 22. Similarly dimensioned spots are obtained on each photoreceptor 24, 26, 28 and 30 by establishing similar optical path lengths for each beam. The laser diodes in U.S. Pat. No. 5,243,359 are arranged in the slow scan direction (i.e., sagittally offset). Diodes arranged in the slow scan direction must be arranged such that they are packed closely in a direction parallel to the polygon mirror's rotational axis to minimize beam characteristic deviations such as spot size, energy uniformity, bow and linearity. Thus, the laser diodes are kept as closely as possible in the direction parallel to the polygon mirror's rotational axis so that the light beams strike nearly the same portion of the polygon mirror as possible.
U.S. Pat. No. 5,341,158 to James Appel et al., the disclosure of which is incorporated herein by reference, discloses a ROS system in which the laser beams are tangentially offset (i.e., separated in the fast scan direction) to offset the laser diode spacing constraints of U.S. Pat. No. 5,243,359 to Fisli.
In single spot rotating polygon based optical systems, bow distortions occur from the accumulation of optic tolerances. Bow is the curved line described by the scanned laser beam of the ROS as the laser beam moves in the fast scan direction. Bow appears as a displacement of the scan line in the process direction as the line extends in the fast scan direction.
Although multi-beam, laser diode based ROS is viewed as the most powerful technology for high quality xerographic printing, differential scan line bow remains an undesirable side effect. Differential scan line bow rises from the very nature of multi-beam optic systems, where the beams are offset sagittally (i.e., in the slow scan direction). The bow occurs because the magnification of the optical system varies across each sagittal plane as each of the sagittally offset beams propagate through the optical system.
Depending on the design of the system, the differential scan line bow can cause scan lines to move towards each other (barrel distortion) or away from each other (pincushion distortion). In both cases, the light sources are on opposite sides of the optical axis. Therefore, the centers of curvature of the bowed scan lines are also on opposite sides of the optical axis.
FIGS. 5A-5C show the problems of differential scan line bow on the surface of a photoreceptor. The ideal scan line 70 is shown as a dashed line. In FIG. 5A, the bowed scan line 72 results from the scanning of the laser beam off the optical axis. When differential bow is not corrected, a printed line may be curved rather than straight. Such errors are generally visible to the human eye.
FIG. 5B shows differential bow in a dual spot ROS. As is well known in the art, a dual spot ROS simultaneously images two spots on a single photoreceptor. In such a ROS, two types of differential bows may occur. In FIG. 5B, a second scan line 74 is created that has a radius of curvature different from the radius of curvature of the first scan line 72. In FIG. 5B, the centers of curvature of the bowed scanned lines 72 and 74 are on opposite sides of the ideal scan lines 70 and create a pincushion distortion.
FIG. 5C shows the bowed scan lines 76 and 78 having centers of curvature on opposite sides of the ideal scan line 70 to create a barrel distortion.
U.S. patent application Ser. No. 08/174,917 to Tibor Fisli et al., filed Dec. 29, 1993, the disclosure of which is incorporated herein by reference, provides a multi-beam ROS in which the chief exit rays from the optical system to the photoreceptor are telocentric. By providing telocentric chief exit rays, the multi-beam system becomes both tolerant to pyramidal polygon angular errors and is able to maintain adequately stable, essentially no bow performance over an acceptable depth of focus in the single station xerographic printer. In addition, by closely controlling the overall shape and orientation of the bow, single pass, multi-station systems are able to print with acceptable levels of misregistration between the various images written on the widely separated xerographic stations.
FIG. 2 shows a top view of a prior art apparatus for mounting mirrors 22 in a ROS system such as that of FIG. 1. Each mirror 22 is mounted such that the reflective front surface 23 faces the incoming light beam while the rear surface 25 faces an opposite direction. The holding members 34 are securely attached to a mounting board (not shown in FIG. 2). The holding members 34 may include arms 32 that extend into contact with longitudinal ends of the mirror 22. The mirror 22 may be attached to the arms 32 using attaching clips, cement or other types of attaching devices as is well known in the art. However, the mirror 22 in an undeformed mode is unable to correct any differential bow problems.